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Hi, I'm a high school student doing a project. How storage temperature affects the pH levels in orange juice?
Question Date: 2010-10-16
Answer 1:

The short answer is that cold orange juice will have a higher pH (be less acidic) than room temperature orange juice. Similarly, warm orange juice will have a lower pH (be more acidic) than room temperature orange juice.

Here is why:
Orange juice, which generally has a pH around 3.5, is acidic because it contains citric acid. Citric acid is a weak acid which means that it does not completely dissociate in water. The behavior of weak acids can be described by an equilibrium constant, Ka. Usually Ka is reported as pKa, where pKa = -log(Ka). The pKa for citric acid is 3.13.

Any weak acid, HA, when added to water will partially dissociate into the conjugate base, A-, and H+ (or more accurately H3O+). This reaction is described by the following equilibrium equation:HA -> A- + H+.The equilibrium constant for this equation is:Ka = [A-][H+]/[HA]

And the pH is given by:
pH = -log[H+]

The equilibrium constant, Ka, essentially tells us how much of HA will dissociate and produce H+. The bigger Ka (or the smaller pKa), the more H+ the acid will produce. In order to figure out how the pH changes with temperature, we need to figure out how Ka changes with temperature.

The vant Hoff equation (which is derived from thermodynamics) tells us that the change in Ka with temperature depends on the enthalpy of the reaction.

lnK2 - lnK1 = - H/R *(1/T2-1/T1)

In the above equation, K1 and K2 are the equilibrium constants, R = 8.314 J/mol K is the gas constant, T1 and T2 are the initial and final temperatures, and H is the enthalpy of the reaction.

According to the National Institute of Standards (NIST), the enthalpy of dissociating in water (called the enthalpy of ionization) for citric acid is 4.07 kJ/mol.

If we set K1 = Ka and T1 = room temperature (25C or 298K), then we can pick different values of T2 and see what happens to K2. K2 will be the equilibrium constant at temperature 2.

First we rearrange the equation:

lnK2 = lnK1 - H/R *(1/T2-1/T1)
If we pick T2 = 1C = 274K (just above the freezing point of water), then we findlnK2 = ln(0.00074) [(4070 J/mol)/(8.314 J/mol K)]*[(1/274K) 1/298K)]lnK2 = -7.21- 0.144 = -7.354K2 = 0.000641
pK2 = 3.19

Since K2 is less than Ka (and pK2 is greater than pKa), then the acid will not dissociate as much when the solution is colder. That means there will not be as much H+ present in the solution and the pH will be higher.

Now if we pick T2 = 37C = 310K (about the temperature of the human body), then we findlnK2 = ln(0.00074) [(4070 J/mol)/(8.314 J/mol K)]*[(1/310K) 1/298K)]lnK2 = -7.21- (-0.0636) = -7.15K2 = 0.000788
pK2 = 3.10

Since K2 is more than Ka (and pK2 is less than pKa), then the acid will dissociate more when the solution is warmer. That means there will be more H+ present in the solution and the pH will be lower.

An easier, but much less accurate way to find the pKa at a different temperature is to use the tabulated value of -0.002 pKa/T. This means that for every 1C (1K) increase in temperature, the pKa of citric acid will decrease by approximately 0.002.

What all of this means for the overall pH is that, although it does depend on temperature, there will only be very small changes. It will not change by more than approximately 0.04 in the temperature range discussed here. Assuming that the orange juice starts with a pH of 3.5 at room temperature, it will stay between 3.46 and 3.54 over the temperature range described above.

Of course, orange juice is not purely citric acid so these calculations for citric acid only give us an estimate for what will happen to the pH of orange juice. The pH should follow the same trend as predicted here, but the numbers might not be exactly right.

Also, keep in mind that this is not true for all acids. Whether the pH increases or decreases ultimately depends on the value of H. If H for the given acid is positive as in the case of citric acid, then that acid will follow the same trend as citric acid. However, if H is negative, then the pH will show the opposite behavior. For these acids with negative enthalpy, pH will increase with increasing temperature and decrease with decreasing temperature.

If you try to measure the pH change of orange juice with changing temperature with a pH meter, you will measure a much bigger change than what is predicted here. That is because the electrical response of the pH meter also depends on temperature so it is only calibrated pro

Answer 2:

The storage temperature does affect the pH levels, but maybe not at a perceptible level.The reason why the temperature affects the pH is from Le Chatlier's principle for chemical equilibria. If you have an solution in equilibrium, such as the orange juice, there is a forward reaction and a reverse reaction happening at the same time. In this case, the reaction that I am describing is the dissociation of citric acid:

Citric acid <=> Citrate + H+

As citric acid dissociates, citrate and a proton are produced. The increased proton concentration decreases the pH. According to Le Chatleir, the equilibrium should shift in a manner to reduce an applied stress. The dissociation reaction above is endothermic, meaning that it requires energy to move forward. So, by increasing the temperature, the reaction moves forward releasing protons, and the solution pH will decrease. If you decrease the temperature, the equilibrium should move to the left and increase the pH.

I hope that this is helpful. Please reply if unclear.


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